Researchers have sped up the stimulated Raman spectroscopy imaging process to make real-time imaging of surface tissue cells practical.
Label-free tissue imaging of a sebaceous gland wrapping around a hair in the viable epidermis of mouse skin. Image in red shows lipid-rich gland cells; green image shows new protein-rich structures such as a hair in the center; blue shows inverse image contrast from the gland.
Still images work for some biomedical applications, but video provides scientists with real-time imaging of drug diffusion in tissues, among other uses. Researchers have now speeded the stimulated Raman spectroscopy (SRS) imaging process to make real-time imaging of surface tissue cells practical (Science 330, 1368).
The technique could make optical biopsies and drug-diffusion studies possible without the use of fluorescent labels, said X. Sunney Xie, professor of chemistry and chemical biology at Harvard University (U.S.A.).
SRS microscopy has gained much attention in the last few years. Xie's group previously demonstrated the technique for still images in vivo (OPN, December 2009, p. 30). By cutting image-acquisition times from approximately 1 min. to as little as 37 ms, the team has now extended the technique to video-rate imaging.
Raman scattering microscopy uses two excitation laser beams focused on the sample of interest. Biological tissue scatters much of the light from the pump and Stokes beams, making the Raman signal weak and extending image-acquisition times.
However, the team designed a way to capture the backscattered light more efficiently: They put the photodetector in front of the lens that focuses the laser light onto the sample. The beam shines through a small aperture in the detector.
Xie said the SRS technique is an improvement over coherent anti-Stokes Raman scattering (CARS) microscopy, which tends to introduce image artifacts that interfere with quantitative analysis.
SRS imaging could be used together with other techniques, such as magnetic resonance imaging (MRI), to determine the type of tumors and to probe their margins, Xie said. Although MRI can penetrate deeper into live tissue, it cannot resolve individual cells the way SRS can.
The SRS process penetrates about 115 µm into tissue, enabling the Harvard group to image the diffusion of a common solvent, dimethyl sulfoxide or DMSO, through mouse and human skin cells. However, according to Xie, surgeons could use the process during surgery as an optical biopsy to help decide where to stop removing potentially cancerous tissue. Xie now believes that the SRS technique will make CARS obsolete.
Patricia Daukantas is a freelance science writer who specializes in optics and photonics.